In the related art, a check valve for preventing a backflow of a fluid flowing from a primary side to a secondary side in a line, such as a pipe, through which a fluid flows has been known. Such a check valve has a valve body and a valve seat which can come into contact with the valve body. The valve body comes into contact with the valve seat by being urged from the secondary side to the primary side toward the valve seat.
When the fluid flows from the primary side to the secondary side, the valve body is separated from the valve seat against an urging force by a pressure of the fluid to allow the fluid to flow from the primary side to the secondary side. On the other hand, when the fluid is to flow from the secondary side to the primary side, the valve body comes into contact with the valve seat by the pressure of the fluid and the urging force, and blocks the flowing of the fluid from the secondary side to the primary side.
Incidentally, when foreign matter mixed in the fluid enters between the valve body and the valve seat, a contact operation of the valve body to the valve seat is inhibited by the foreign matter. Therefore, in a situation where the fluid is to flow backward from the secondary side to the primary side, a gap is generated between the valve body and the valve seat, and the backflow of the fluid cannot be completely blocked in some cases. Accordingly, a check valve capable of detecting entry of such foreign matter (dust) has been proposed (for example, see Patent Document 1). In addition, a technique has been proposed in which two check valves are disposed in series so that even if the contact operation of one check valve is inhibited by the foreign matter, the contact operation of the other check valve can block the backflow (for example, see Patent Document 2).